It is necessary to maintain the integrity of structural objects and components that have been subjected to wear and stress from use and hazards of the environment in which such objects and components operate. Accordingly, it is desirable to test such structural objects and components periodically to determine whether they have degraded in strength or reliability due to such flaws as surface cracks, corrosion, disbonds, and the like. In some instances, it is possible to remove the object to be tested from its location of use and perform a test of its integrity while it is mounted on a test stand in a laboratory using laboratory instruments. However, in many instances the test object is very large or is integrated into a larger structure in a manner that make its removal impracticable, making it difficult if not impossible to remove it for remote testing.
In addition, it is desirable to perform nondestructive evaluation (NDE) tests on objects. NDE tests do not permanently alter an object in an undesirable manner, which may render the test object useless for its intended purpose.
Nondestructive evaluation systems and methods have been developed to provide non-contact inspection of components and structures in the field to detect flaws and otherwise determine the integrity of such structures and components. For example, techniques have been developed for infrared or thermal imaging of subsurface defects in a material that are illuminated by ultrasonic or sonic vibrations. A sound source generates sound waves in the test object by pulses of energy having a constant frequency and amplitude for a predetermined period of time.
The sound source can be an electromagnetic acoustic transducer (EMAT) that provides broadband, pulsed ultrasonic energy. A thermal imaging camera or device is used to image the test object as it is being excited by the EMAT sound source. A sequence of images is recorded that may show cracks and other defects in the test material, which appear as light areas against a darker background. The images may be displayed on a monitor, and a storage device may be provided to store the sequence of images to be reviewed at a later time.
Disadvantages with such techniques for infrared or thermal imaging of subsurface defects in a material arise because of the need to fix the distance between the source of sound waves—the EMAT—and the infrared imaging device so that the propagation of sound waves in the test object may be properly synchronized between the transmitter of the sound waves and the imaging device. Further, there is a need for providing such nondestructive evaluation systems that may operate on relatively large objects and that may inspect large objects in a short amount of time.